The present invention relates to a long-distance optical communication system employing multiple optical amplifiers, and more particularly relates to a long-distance high-speed optical communication scheme in which an optical signal can be transmitted at a high speed.
Since an optical amplifier such as a semiconductor laser amplifier, which directly amplifies an optical signal as it is in the form of light, is simple in constitution and does not have restrictions on the speed of transmission, there is an advantage that a very-efficient long-distance optical communication system can be constituted by combining the optical amplifier with a single-mode optical fiber.
FIG. 10 is a block diagram of a (conventional) long-distance optical communication system employing a large number of optical amplifiers. Shown in FIG. 10 are a light source 1 such as a semiconductor laser, optical fibers 2a, 2b, . . . and 2n for transmitting optical signals La, Lb, . . . and Ln, the optical amplifiers 3a, 3b, . . . and 3n for directly amplifying the optical signals La, Lb, . . . and L.sub.(n-1), and a receiver 4 which demodulates the optical signal Ln. Digitally communicated information DI is converted into the optical signal La by the light source 1. The optical signal La is sent into the optical fiber 2a. The optical signals La, Lb, . . . and L.sub.(n-1) attenuated while being transmitted through the optical fibers 2a, 2b, . . . and 2.sub.(n-1) are amplified by the optical amplifiers 3a, 3b, . . . and 3n provided at the large number of stages with nearly the same interval between the optical fibers 2a, 2b, . . . and 2n, so that the digitally communicated information DI arrives at the receiver 4 through a multiple repeaters. The digitally communicated information DI is thus transmitted through the communication system. As shown in literature such as "904 km 1.2 Gbps Non-regenerative Optical Transmission Experiment Using 12 Er-doped Fiber Amplifiers" by N. Edagawa et al., PDA-8, ECOC89", it has already been confirmed experimentally that it is basically possible to transmit a high-speed signal in such a transmission scheme.
Studies have been intensively made in order to transmit an optical signal at a high speed through a long-distance optical communication system employing optical amplifiers. As a result, various proposals have been made. For example, it has been proposed that the wavelength of an optical signal is set at 1.55 .mu.m at which the loss of a silica-based optical fiber is minimum, and the fiber is modified as a dispersion shifted optical fiber to avoid a dispersion which would be a problem in transmitting a high-speed signal. However, a long-distance optical communication system having a transmission speed of 1.2 gigabits and a transmission length of about 1,000 km and according to a conventional long-distance optical communication method employing optical amplifiers has been only reported to be the longest-distance optical communication system. That system is far insufficient to be a transoceanic optical communication system of about 10,000 km in length. Accordingly, a long-distance high-speed optical communication in which an optical signal expected to be transmitted at a higher speed in the future can be transmitted at that speed through a long-distance optical communication system employing optical amplifiers has been strongly desired, but not disclosed.